Fabrication of materials by high energy-rate impaction



Sept. 26, 1967 J. R. HAGUE ETAL 3,344,209

FABRICATION OF MATERIALS BY HIGIi ENERGY-RATE IMPACTION Filed Dec. 5-, 1966 Inventors James R. Hayde Daniel. ZZL. Brita flifirflgy United States Patent 3,344,209 FABRICATION OF MATERIALS BY HIGH ENERGY-RATE IMPACTION James R. Hague, Golden, Colo., and Daniel W. Brite,

Richland, Wash., assignors to the United States of America as represented by the United States Atomic Energy Commission Filed Dec. 5, I966, Ser. No. 599,681

6 Claims. (Cl. 264-.5)

This invention was made in the course of or under a contract with the United States Atomic Energy Commission.

This invention relates to a method of forming bodies from powdered material by means of high energy-rate impaction. It is directed specifically to a method which avoids the cracking of brittle materials formed in this manner.

The formation of bodies from powdered material by high energy-rate impaction has been described in a number of publications, e.g. U.S. Patent 3,276,867, granted October 4, 1966 to Daniel W. Brite and Kenneth R. Sump, and Transactions of the American Nuclear Society, vol. 7, No. 2, November 1964, pp. 408 and 409. According to this method, powders are confined in a container which after heating and evacuation is placed within a rigid die which confines it on all sides except the top. It is then impacted by a heavy rapidly moving ram. Typically,the' ram may weigh about a ton and travel at approximately 35 to 75 feet per second depending on the pressure desired. Under these conditions, the powder is compressed into a solid body. When the impacted material is brittle, it is frequently found on removing the can that the body is severely cracked. In some cases, as when the material is to be crushed, the cracking is immaterial. Frequently, however, it is desired to retain the compact in the approximate size and shape in which it is formed. This invention makes it possible to accomplish this objective even when the material is brittle, e.g., beryllium oxide or a mixture of beryllium oxide and uranium oxide.

The cause of the cracking is somewhat uncertain. Since the brittle materials are relatively strong in compression but weak in tension, it is believed that the cracking occurs on expansion on rebound of the ram after impact, but thermal effects are probably also involved.

We have found that the cracking can be eliminated provided the powder is contained in a graphite can, which is surrounded by a granular material, e.g. magnesium oxide, which in turn is contained in an outer impaction vessel. We believe that this arrangement prevents the development of tensile stresses during expansion.

The arrangement is shown in the drawing which is a cross section through the vessels and the surrounding die.

In this figure the powdered material to be impacted is shown at 1, the graphite can at 3, provided with a loosely fitting cover 4, the granular packing at 5, the impaction container at 6, and the surrounding die at 7. The container and contents are heated and evacuated through a tube which is then sealed off at 8. The upper end, 9, is impacted by a punch carried by the rapidly moving ram (not shown).

Container 6 rests on a movable plug 10. After impaction, plug and container 6 are forced out of die 7 by a punch 11, which operates through bore 12.

The powder should be loaded in the graphite container at a bulk density which is at least 50% of the theoretical density of the material. This is necessary in order to prevent crumbling of the graphite on impact. Preliminary compaction may take place directly in the container, or the powder may be pressed into pellets which are of the proper size to fit the container. The loosely fitting cover 3,344,209 Patented Sept. 26, 1967 rests on the compacted powder. It permits gases to escape during evacuation.

The combination of the granular packing 5 and the graphite can 3 appears to be necessary to achieve satis factory results. The granular packing apparently acts as a pseudo-hydraulic medium and distributes the pressures evenly about the graphite can 3 and its contents. The graphite is compressible and collapses inwardly upon the compacted powder, while still retaining its general shape. The reduction in dimensions is substantially the same in all directions. It is believed that the combination of these effects plus the thermal insulating properties of the graphite and the granular packing are responsible for the success of our process. We do not however, wish to be bound by this theory, relying rather on the observed results.

We have produced crack-free specimens of BeO and BeOUO These powders after compaction, have been impacted at temperatures of 1500 to 1800 C. and 14,000 to 20,000 kilograms per square centimeter. Specimen sizes have been 1.6 cm. in diameter by 1.7 to 2.4 cm. long. Thickness of the graphite can varied from 0.16 to 0.64 cm. Particulate magnesium oxide and aluminum oxide have been successfully employed as the granular material. The container 6 was 5 cm. in diameter by 10 cm. long and was made of molybdenum.

Specific embodiments of the invention will now be described. The material impacted consisted of BeO and mixtures of BeO with U0 BeO was a submicron powder. The powdered material was pelleted by cold pressing before insertion in the graphite can. No binder was used.

Table I summarizes the pressing data. For the BeO U0 compositions, cold pressing was preceded by mechanically mixing the BeO with spherical U0 particles having a density of 95% of theoretical (-100-l-140 mesh). The ultrafine BeO particles readily agglomerated during blending. and also appeared to coat the U0 spheres.

The pellets were subsequently inserted in small graphite cans, and the assemblies centrally located in larger molyb denum impaction containers (5.080 cm. OD by 0.160 cm. Wall thickness by 10.160 cm. long). Wall thicknesses of the graphite cans were 0.318 cm. for the BeO assemblies and 0.160 to 0.635 cm. for the BeO-UO assemblies. The resultant voids between the graphite and molybdenum containers were then vibrationally filled with powdered MgO or A1 0 and the cans were sealed, heated, evacuated, and impacted under conditions shown in Table II. 7

Visual examination of the impacted BeO specimens revealed no apparent laminar cracking. The pellets had fairly uniform surface characteristics and dimensions. The BeO compacts were subsequently fractured by impact and, again, revealed no internal lamination. The BeO- UO specimens were similarly intact, with the exception of a checkering effect on their surfaces.

Mercury porosimetric densities of the BeO pellets (Table II) revealed no significant variation under pressures resulting in mercury penetration of pores ranging from 100 to 0.06 micron in diameter.

H 0 and CCl displacement densities of these specimens were approximately all 99% TD (theoretical density) indicating that the lower density samples (83 and 85% TD) contained considerable interconnected (open) porosity, the pore size being generally greater than 0.02 micron in diameter (limits of pore size penetration by H O or CCl The absence of extensive open porosity in the denser BeO specimens indicates more complete particle bonding, presumably achieved through the development of higher impact pressures.

Within the limits of accuracy of the U0 distribution (25:10 vol. percent), the bulk densities of the BeO U0 specimens were all greater than of theoretical,

3 with no appreciable difference in density noted for variations in impaction conditions.

It will be understood that the conditions may be varied, particularly when the process is applied to different materials. For example, uranium dioxide or plutonium dioxide or mixtures of these may be satisfactorily compacted at temperatures of 1200 to 1300 C. Under these conditions stainless steel may be substituted for molybdenum as the material for the outer can 6. The outer can must be made sufiiciently strong to resist bursting on rebound of the punch, since it has been found that there is a sudden expansion of the powdered material 5 at this point in the operation.

centimeter.

6. A process as defined in claim 3 wherein said impact takes place at a temperature of 1500 to 1800 C. and at TABLE I.COLD-PRESSING DATA FOR BeO AND BeO- VOL. PERCENT U02 Pellet Pellet Pellet Pellet Pressure, Pellet Material No. wt., g. diam., cm. length, kg./cm. density,

cm. percent TD BeO 1 6. 4 1. 588 1. 969 2, 275 BeO 2 6.3 1. 600 1.969 2, 240 53 BeO 3 8. 0 1, 588 2. 413 2, 275 54 BeO 5 6.1 1. 631 1, 854 2, 205 54 Be0-25 vol. percent U 1.. 3 11.9 1. 600 1.803 4, 480 66 BeO-25 vol. percent U02 4 11.8 1. 600 1.816 4. 480 BeO-25 vol. percent U0 5 10.9 1. 600 1. 727 4, 480 63 l BeO: 3.01 g./cm. BeO-25 vol. percent U0 4.99 g./em.

TABLE II.PNEUMATIC IMPACTION DATA FOR BeO AND BeO-25 VOL. PERCENT UOz Pellet 1 Ceramic 2 Preheat Impact Pellet 4 Material N0. oxide filler temperature, pressure, density,

C. kgJcm. percent TD BeO 1 1, 510 14, 280 83 BeO 2 1, 600 15, 400 85 BeO 3 1, 700 18, 480 99 BeO 5 1, 500 17, 500 96 BeO-25 vol. percent U02 3 1, 800 19, 530 BeO-25 vol. percent UOL 4 1, 675 15, 960 95 BeO-25 vol. percent U02 5 1, 700 180 95 1 All BeO pellets and BeO-UO; pellet No. 3 were inserted in graphite containers having a wall thickness of 0.318 cm. The corresponding graphite wall thicknesses were 0.635 and 0.160 cm. for BeO-UO; pellet Numbers 4 and 5, respectively.

2 Metco alumina spray powder, 40 microns particle size, calcined MgO, 75 to 100 microns particle size.

a 15 min. normal duration.

4 BeO: 3.01 g./cm.-'*; BeO-25 vol. percent U02: 4.99 g./cm. Densities of the BeO specimens determined by mercury prosimetry; densities of the BeO-25 vol. percent U02 specimens determined by weight and dimensional measurements.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a process in which a powdered material is formed into a solid mass by enclosing it in a first container, confining said container in a die so as to prevent expansion, and subjecting said container to a violent impact, and wherein said solid mass is brittle; the improvement comprising: enclosing said powdered material in a second container within and spaced on all sides from said first container, the space between said containers being filled with granular material and said second container being made of graphite, whereby cracking of said solid mass is avoided.

a pressure of 14,000 to 20,000 kilograms per square centimeter.

References Cited UNITED STATES PATENTS 50 2,725,288 11/1955 Dodds et al. 264.5 X 3,276,867 10/1966 Brite et a1. 75-226 X 3,279,917 10/1966 Ballard et al. 75--226 3,384,195 11/1966 Googin et a1. 75-226 55 L. DEWAYNE RUTLEDGE, Primary Examiner. 

1. IN A PROCESS IN WHICH A POWDERED MATERIAL IS FORMED INTO A SOLID MASS BY ENCLOSING IT IN A FIRST CONTAINER, CONFINING SAID CONTAINER IN A DIE SO AS TO PREVENT EXPANSION AND SUBJECTING SAID CONTAINER TO A VIOLENT IMPACT, AND WHEREIN SAID SOLID MASS IS BRITTLE; THE IMPROVEMENT COMPRISING: ENCLOSING SAID POWDERED MATERIAL IN A SECOND CONTAINER WITHIN AND SPACED ON ALL SIDES FROM SAID FIRST CONTAINER, THE SPACE BETWEEN SAID CONTAINERS BEING FILLED WITH GRANULAR MATERIAL AND SAID SECOND CONTAINER BEING MADE OF GRAPHITE, WHEREBY CRACKING OF SAID SOLID MASS IS AVOIDED. 